1. Field of the Invention
The present invention relates to systems and methods for reading data stored on a storage medium, and, in particular, to multipath data reading systems for throughput sensitive applications, in which multiple data paths are used to simultaneously read data.
2. Description of the Related Art
There is often a need to read data stored on a storage medium or device, such as hard disk drives, optical disk drives such as CD-ROMs (compact disk-read only memory), floppy diskettes, tape drives, or other types of computer-readable storage media or devices, or data received from a communications system. The data is typically embodied in a data channel transmitted from a data source to the data reading system.
In such storage devices, data is stored in the form of signals that represent data bits, in a format that depends on the particular medium. The bits are typically stored in distinct physical portions or locations of the medium, such as individual sectors of given tracks, for hard drives. Each sector or location may be considered to store a "message" containing a finite number of bytes or bits of data. Raw data is typically read from the storage medium by some type of transducer to provide a "data channel" from which data is then read, by various techniques, with a data read or recovery system.
On a hard drive, data bits are stored in track sectors. A magnetoresistive (MR) head (i.e., the transducer) close to the surface of the spinning disk of a hard drive typically provides a head signal, from which specialized circuitry or other systems attempt to read data messages or sectors of data originally stored on the drive. Each data message to be read has a characteristic, or type. For example, in the above-described case the characteristic of a given data message is the sector number or location on which it was stored.
Two aspects of data storage devices are under constant pressure for enhancement. One is the continuing effort to increase storage density, and the other is the continuing effort to increase throughput by using higher speed components and improved techniques. For example, with respect to hard drives, storage density is improved by increasing the bit densities and by narrower, more closely-spaced data tracks. These changes result in a reduction of the size of each bit, which requires that the reading and writing capabilities be upgraded to higher levels of performance to maintain the same standard of reliability previously attained.
Among the characteristics that are designed to optimize drive performance during the read mode is delta-V, which is the voltage change per unit time threshold that is used to discriminate between data and noise signals on the data channel produced by the transducer. It has been common practice to set the delta-V value for optimum performance. However, the delta-V value that provides the best performance varies from transducer to transducer and, in a disk drive, also with the radial position of the transducer location, which is a function of the speed of the head relative to the disk surface. U.S. Pat. No. 4,821,125 (the '125 patent) addresses this problem with disk drive channel circuitry, which uses variable values for delta-V that can be set on a head-to-head and track-to-track basis. Such channel circuitry uses only a single delta-V value at a time, however, and thus requires data to be re-read for each delta-V value when errors are encountered. This disadvantageously decreases throughput because the disk must be rotated into the transducer location so that data can be re-read from the disk.
Another characteristic that is designed to optimize drive performance during the read mode is the timing window in which a bit is read from a medium. It has been common practice to center the window for optimum performance using variable frequency oscillator (VFO) window centering circuitry. However, the position of the timing window that provides the best performance can vary due to bit shift, which is caused by, for example, different transducer/disk combinations and tolerances in the VFO window centering circuitry. To address this problem, disk drive channel circuitry is known that uses a variable position timing window that can be shifted either early or late. For example, U.S. Pat. No. 4,958,243 and "Window-shifting Mechanism in Data Separator," IBM Technical Disclosure Bulletin, Vol. 30, No. 6, November 1987, disclose typical channel circuitry of this type. Such channel circuitry shifts the timing window for a whole data string, however, and thus requires data to be re-read for each desired shift of the timing window. This disadvantageously decreases throughput because the disk must be rotated into the transducer location so that data can be re-read from the disk.
Decreases in throughput are especially disruptive in applications where uninterrupted read data flow is of great significance, such as multimedia or other video applications. Decreased throughput in multimedia applications manifests itself in jerky visual motion or stopped scan updates in mid-screen while data is re-read, since there is a latency caused by re-reading a sector of data. When such latency is incurred in re-reading a sector of data in order to successfully read the data, the data reading and re-reading steps are sometimes referred to as data recovery. When no extra latency is incurred, i.e. when the data recovery system is able to successfully or adequately read the data from the data channel without a re-read, the data is simply "read" but not "recovered." A given data read system may be referred to as a data recovery system since such systems typically have the ability to "recover" data if necessary, i.e. if the first attempt at reading was unsuccessful.
One conventional approach employs a system, described in U.S. Pat. No. 5,557,482, col. 2, that compensates for bit shift driven by intersymbol interference, i.e., the first and last changes in a plurality of successive changes in the direction of magnetization will upon reading respectively result in an early-occurring and late-occurring data bit frame. This system uses one unique path for detection delay, one detection for framing data without any delay and another detection for framing the data with a delay equal to the anticipated intersymbol interference driven bit shift. Framing is strobed by a clock that is delayed a fraction of the anticipated intersymbol interference driven bit shift. The framing path is selected based on an inflexible set consecutive bit criteria, i.e., whether the bit that was read last corresponds to a change in the direction of magnetization or no change in the direction of magnetization. This system only compensates for intersymbol interference-driven bit shift using the unique set consecutive bit criteria rules.
Undesirable latency can also occur when reading or recovering data received from a data source other than a hard drive. For example, in the case of data received from other types of storage medium or from a communications system, the data source may also be requested to re-transmit a chunk of data when an error is detected.
U.S. Pat. No. 5,557,482, issued to Christensen et al. on Sep. 17, 1996, for "Multipath Channel Apparatus and Method for Data Storage Devices and Communications Systems Wherein a Data Path Is Selected Based on Errors" (the '482 patent), the entirety of which is incorporated herein by reference, describes a multipath channel apparatus for a data storage device wherein data is stored on a storage medium and is read from the storage medium by a transducer, or for a data communications system wherein data is transmitted to a receiver. In the '482 patent, a data channel having a plurality of data paths is operatively connected to the transducer to receive data read from the storage medium by the transducer, or operatively connected to receive data demodulated by the receiver or base band data received by the receiver. Preferably, each of the data paths has a differing parameter value, or different sets of parameters that define and characterize the operation of various components of each data path. An error checking unit checks customer and redundancy data from each of the data paths. A selecting unit, which is responsive to the error checking unit, selects data from one of the data paths based on a favorable error correcting code (ECC) syndrome for the data (typically a chunk of data such as a sector of data in the case of a hard disk drive). Thus, by using multiple data paths, the chance of successfully reading the data in the data channel by at least one of the data paths is increased.
Traditional magnetic recording channel designs utilize a single "path" for the data between the head and the decoded data standardizer, where a path provides certain processing, filtering, detection and/or standardization functions for reads of data applied thereto. As explained above, systems utilizing a multipath design can have enhanced data read techniques, which are especially useful in throughput-sensitive applications such as multimedia. However, even in a multipath system, if data is not read successfully the first time the transducer reads data from a given sector of a given track, the necessity to re-read or engage in other data recovery procedures (DRP) can cause interruptions that are extremely detrimental, particularly in throughput intensive situations such as video or multimedia applications.